Water-swellable polymers are used as constituents in sanitary materials which absorb body fluids such as: sanitary napkins, incontinent devices and disposable baby diapers. Examples of such polymers are disclosed in U.S. Pat. Nos. 3,926,891: 4,190,562: and 4,293,609, herein incorporated by reference.
Various processes are known for producing absorbent polymers. For example, U.S. Pat. No. 4,833,222, herein incorporated by reference, teaches a process for preparing an absorbent polymer from a neutralized monomer without a surface-active agent. U.S. Pat. No. 4,808,637, herein incorporated by reference, teaches the uniform reaction of acrylic acid, an alkali metal salt of carbonic acid, aluminum acetate, sodium persulfate and water, preferably using microwave radiation as the heat source to initiate polymerization.
A high level of residual monomer of water-soluble polymers is of concern in such materials because the extractable polymers may be leached from the absorbent structure by aqueous fluids. A high level of water-solubles can cause low water-absorbing capacity in the adsorbent device. Thus, when these water-swellable polymers are contacted with catamenial blood, urine, or other body fluids, they have low initial absorption rates, less effective absorption and become sticky. Therefore, a high water-soluble polymer content can inhibit the absorptive capacity of the absorbing device.
U.S. Pat. No. 4,920,202, also incorporated herein by reference, teaches a method for drying a hydrated polymer gel that leads to relatively low residual monomer levels in the dried absorbent polymer. The process requires contacting the gel polymer with a gas containing at least steam and having a dew point in the range of 50.degree. C. to 100.degree. C. at a temperature in the range of 80.degree. C. to 250.degree. C. The use of high dew point air for drying a hydrated polymer gel results in substantially reduced dryer throughput rates. Therefore, it is desirable to provide an improved process for reducing residual monomer content while drying hydrated polymer gels, which process does not require the use of circulating high dew point air.